1. Field of Invention
The present invention relates to an earphone detection circuit. More particularly, the present invention relates to an earphone detection circuit without a common ground terminal for left and right audio channel.
2. Description of Related Art
At present, most audio signal providers such as audio recorders, camcorders, televisions or portable computers have two major audio output channels. Aside from having a built-in speaker, these audio signal providers also have a socket for plugging an earplug so that people may listen through an earphone. In addition, these audio signal providers have an automatic detector inside for switching the audio attendance mode automatically. In other words, audio signals are channeled to the earphone or other externally plugged device once the earplug is plugged into the socket. Conversely, if the socket is unoccupied, audio signals will be re-routed to built-in devices such as a pair of speakers.
However, to provide a function for the automatic switching of output pathways, a suitable earphone driving circuit must be present so that the occupation of an earphone (or other output device) can be detected. FIG. 1 is a conventional earphone driving circuit and corresponding earphone detection circuit. In FIG. 1, the left and right audio channels are amplified through amplifiers 102 and 104 respectively. Thereafter, the direct current (DC) components of the amplified signals are filtered through capacitors 106 and 108. Finally, the signals are passed to the earphone through contact points 118 and 120 respectively. In the absence of an earplug inside the socket, contact point 124 and the audio signal transmission line 112 are in contact. Since the resistance of the resistor 122 is a lot higher than the resistance of the resistor 126, a detector 130 is able to detect a zero voltage from a detection line 132. In this way, the detector 130 will correctly determine the absence of an earplug inside the socket. Consequently, transmission of audio signals via the earphone driving circuit is prevented. On the other hand, if an earplug is plugged into the socket, contact point 124 is forced away from the audio signal transmission line 112. Thus, the detector 130 will receive a voltage of about Vcc. Again, the detector 130 will correctly determine the presence of an earplug. Ultimately, the audio signal is transmitted through the earphone driving circuit
Although everything seems fine with this circuit arrangement, the capacitors must have a large capacitance and hence tend to occupy a large volume. This is because a larger capacitance is needed to produce a better frequency response. Thus, reducing overall volume of the earphone driving circuit is difficult.
To resolve the bulky capacitor problem, an earphone driving circuit without any capacitor as shown in FIG. 2A is developed. As shown in FIG. 2A, since potential at a central point serves as a common ground terminal, an earphone detection circuit 230 composed of the resistors 122 and 126 and the detection line 132 as shown in FIG. 1 cannot be used. Therefore, a special earphone detection circuit 230 suitable for an earphone driving circuit is required.
In brief, the conventional technique for detecting the presence of earphone either contains bulky circuits or is not very effective.